Logarithm of europium concentration



April 6, 5 R. R. SODEN ETAL 3,177,155

OPTICAL MASER. CRYSTALS Filed Aug. 7. 1961 FIG.

, CURVE 2 m I-Ll cunw: 2 5

l l E LOGARITHM OF EUROPIUM CONCENTRATION 3 LG. VAN U/TERT BY UnitedStates Patent 3,177,155 OPTECAL MASER CRYSTALS Ralph R. Soden, ScotchPlains, and Le Grand G. Van Uitert, Morris Township, Morris County,N..l., assignors to Bell Telephone Laboratories, Incorporated, New

York, N.Y., a corporation of New York Filed Aug. 7, 1961, Ser. No.129,793 3 Claims. (Cl. 252-3015) This invention relates to singlecrystal tungstate and molybdate materials exhibiting fluorescentproperties and to devices utilizing such crystals.

Recently, considerable interest has developed. in a new class of solidstate maser devices in which the stimu lated frequency is in the opticalor near optical spectrum including the infrared and ultraviolet portionsof the electromagnetic spectrum. This spectrum encompasses thewavelength range of :from 100 A. to 2x10 A. In principle, these devicesare directly analogous to the microwave maser, and the mechanics oftheir operation are well detailed in the literature, for example, asdescribed by A. L. Schaw-low and C. H. Townes in US. Patent 2,929,922,issued March 22, 1960.

Among the more promising forms of optical masers are those which employa material whose energy level system is characterized by at least threeenergy levels, with the separation of these levels falling within thedesired operating frequency ranges. During operation, there isestablished, at least intermittently, a nonequilibrium electronpopulation distribution in a pair of the selected three energy levels.In particular, the population of the higher of the selected pairs ofenergy levels is increased to the point at which it is greater than thatof the lower level. It is customary to refer to a material in such astate of nonequilibrium as exhibiting a negative temperature.

It is characteristic that if there is applied to a material in anegative temperature state a signal of a frequency which satisfiesPlancks Law with respect to the 'two energy levels in nonequilibrium,the applied signal will stimulate the emission of radiation in phasewith the signal frequency from the material and the signal will beamplified. In other words, the active maser material is chosen such thatthe two energy levels are separated by an energy equal to h/L, where his Plancks constant and ,u. is equal to the frequency to be amplified.This separation is less than the separation between the top and bottomlevels of the selected three-level energy system.

The negative temperature state is established by applying to thematerial pump energy ofa frequency of at least the frequencycorresponding to the separation be; tween the top and bottom levels ofthe selected'threelevel energy system. The application of sufiicientpump energy affects electron transitions from the bottom level to thetop level and the populations of the bottom and top levels are therebymade to approach equality. Under these conditions there will be anegative temperature either between the top and middle levels or betweenthe middle and bottom levels. Since a competing process known asrelaxation tends to return the system to equilibrium, therebydestroyingthe negative temperature state, continuous pump energy isapplied to the material during the period of signal amplification.

Among the more promising active maser materials are those which comprisea host crystal containing paramagnetic ions from which the stimulatedemission occurs. The host crystal of a material meeting theabove-described requirements must be capable of accepting theparamagnetic ions in such a way that they are able on excitation tofiuoresce with good over-all quantum efliciency, with as much of theemitted energy as possible concentrated in a single line. The maximizeamplification of the signal frequency, the emission line preferablycorresponds to a transition to a state other than the ground state suchthat the single bright emission line is narrow in width.

Since the pump sources typically utilized .in optical masers generallyexhibit an energy output over a broad frequency spectrum, it isdesirable that the paramagnetic ions possess a broad absorption spectrumto facilitate establishment of the negative temperature state. Desirablythe paramagnetic ions also exhibit a relaxation time sufficiently longso that the quantum efiiciency for fluorescence is close to unity.Otherwise, the magnitude of the pumping frequency would have to begreatly increased in order to maintain a negative temperature statewherein sufficient electrons are available in the higher energy level toamplify the input frequency. To ensure a narrow emission line, theenergy level widths of the pair of spaced energy levels in the negativetemperature state are preferably narrow.

In view of the above-detailed requirements, very few optical masermaterials are known to the art. Most published work on optical masers isdirected to ruby crystals and calcium fluoride crystals containing smallamounts of uranium (III) and samarium (II). Ruby crystals, however,suffer the disadvantage of requiring high pumping power to establish anegative temperature state. As such, under the usual conditions rubymasers are limited in operation to producing a pulsed beam of coherentlight.

As previously discussed, there should be a correspondence between thesignal to be amplified and the energy level separations of the masermaterial. Therefore, it is desirable that new maser materials having arange of energy level separations and fulfilling the above-detailedrequirements be developed so that a range of signal frequencies can beamplified.

In accordance with the present invention a new fluorescent compositionof matter suitable for use in optical maser devices has been developed.The host lattices of this composition are potassium-terbium tungstateand potassium-terbium molybdate in which restricted amounts of theterbium atoms have been replaced with europium in the 3+ valence state.The composition has the empirical formula:

where A is selected from the groupconsisting of W0 and M00 and x has avalue of 0.003 to 0.5. The subscripts in the above formula signify therelative number of gram atoms of the element indicated which are presentand thus are also proportional to the relative number of atoms of eachelement present in the composition.

The materials of the instant invention emit energy of narrow line width.For example, the line width of K Tb Eu WO associated with an emittingwavelength of approximately 6150 A. is in the orderof 2.5 cmf at liquidnitrogen temperature. The excited electrons evidence a relaxation timesufiiciently long so that the quantum efiiciency for fluorescence isclose to unity.

Since the ions possess at least three energy levels and FIG. 2, oncoordinatesof relative emission intensity and gram atoms per formula oftrivalent europium ion,

is a logarithmic plot showingthe dependency of the emission intensity onthe concentration of europium ion in the material of the presentinvention.

Referring more particularly to FIG. 1, there is shown a rod shapedcrystal 1 having the composition as disclosed herein. Pump energy issupplied by means of. heli cal lamp encompassing rod land connected toan energy source not shown. Lamp 2 is anultraviolet lamp having acompact arc of high pressure mercury. Ends 3 and 4 of rod 1 are groundand polished so as to be optically flat and paralleland are silvered soas to provide reflective layers 5. and 6. As indicated, layer 6 iscompletely reflecting while layer 5 is only partially reflecting sopermitting the escape of coherent radiation 7, having.

a wavelength of approximately 6150 A. Rod 1 during operation ispreferably maintained in an atmosphere of liquid nitrogen (at a'temperature of approximately 79 K.) so as to more readily attainzanegative temperature state.

The spectrum of the pump source including ultraviolet light is desirablyWithin the range of 2,000 A..to 4,200 A.

Although higher frequencies are suitable, sources of such frequenciesare not generally available. It has been found that ultraviolet lighthaving a peak of 366 A. is'most advantageous for the present purposes.

Although the expressed range is the range of energy most effective, itis not necessary to use a source having an output restricted to thisrange. For example, a gaseous discharge flash bulb, although emittingwhite light, nevertheless emits a large amount of energy in the desiredspectrum. Device discussion has been-largely in terms of the mostcommonly reported maser design. Although such a device is easilyfabricated, other configurations have been disclosed in the literatureand may prove advan- 4." a sharp decreasein the emission intensityexhibited by r the materials of the present invention. Theupper limit of0.5 is obtained when all of the terbium in the potassium-terbiumeuropium tungstate crystal has been replaced by europium, resulting in apotassium-europium tungstate crystal. As seen from curve '1, increasingamounts ofrare earth ioninclusions above the minimum limit of 0.003causes the emission intensity to pass through a maximum and thendecrease until the plotted emission intensity for potassium-europiumtungstate is attained.

Based on the. preceding considerations, a preferred europiumioninclusion range is0.008 to 0.4v gram atom per formula with an optimumrange being 0.01 to 0.2 gram atom per formula} To obtain curves 1 and 2of FIG. 2, measurements Were made on various materials of the presentinvention with a Gaertner high dispersion spectrometer adapted with an.AMINCO. photomultiplier using a IP22 tube. Ten micron slit widths wereemployed at theentrance and exit to the spectrometer; The system wascalibrated against a tungsten filament lamp, whose'output was assumed tohave a black-body dependence, to give relative values of brightnessofthe-emitting surfacein units of power per unitwavelength range. Emissionwas excited by illuminating asample one inchlong by one-half inch wideby one-quarter inch deep with a 3660 A. rich H4 spot'lampthrough aCorning 5874 filter. The measurements on the tungstate crystals .of theinstant invention were made at roomtemperature, and the measurements onthe molybdate crystals of the instant invention were made at liquidnitrogen temperature. The intensities are relativeto 100 for the6l50 A.peak of a comparable sam- P16 Of NZO.5EUO 5WO4. i

As evidenced by curve 1 of FIG. 2, the substitution of potassium forsodium in the Na Eu WO structure restructure having, a uniform internalelectrostatic field I which'permits narrow line width.

tageous. All such variations are considered to be Withthis figure showsthe efiect of europium inclusions in one composition of the inventionhaving the empirical formula K Tb Eu WO It has been found. that replacngW0 in the above formula with M00 results in molybdate compositionshaving line widths and emis-.

sionintensities' comparable to the tungstate crystals of the nvention. II

Based on FIG.2, inclusions of 0.003 to 0.5 gram atoms per formula oftrivalent europium in the tungstate and molybdate host crystal result ina material exhibiting .an enhanced emission intensity. Thelower limit of0.003 1 gram atom is based on the necessity of having sufiicientunpaired, electrons availablegin the negative temperature state toadequately amplify the input signal. As seen from curve 1, smaller rareearth ion inclusions result in As further evidenced-bycurve 1 of thisfigure, terbium ion dilutions of the potassium-europium tungstatestructure enhance the-emissionintensity of this'structure. The peakemissionintensity of the. resulting structureis comparable. inbrightness to that of sodium-europium tungstate.q

It has been determined that. replacing terbium with other rare earthions in the composition of the invention results in either anonfluorescent orweakly fluorescent materiaL; For example, thesubstitution of cerium for terbium results in a nonfluorescent material,while the substitution of dysprosium, erbium, holmium, thulium, andother rare earth ions results in only a .weakly fluorescent material. Ithas further been determined that replacing terbium with othernon-rareearthions in the compositions of the; invention results inamaterial. exhibiting an emission intensity lower'than thepotassiumeuropiurn tugnstateend component. Illustrative of this isplotted curve 2 for potassium-yttrium-europium tugn-- state whichexhibits the highest emission intensity at potassium-europium tungstate.I

The following tables set forth D spacings and emission intensities ofthe compositions of the inventioncalculated from X-ray, diffractionpatterns of the compositions. The X-ray ditfraction wpatterns were taken'with a Straumanis-typeNorelco camera 114.6'rnm. diameter) usingchromium, potassium alpha radiation. Table I sets are not critical andordinary reagent grade tungstates 'or molybdatesor substances that reactto form the tung-' 'is not critical. However, it is well known to use anoxygen-containing atmosphere such as air, oxygen or oxygen plus an inertgas to prevent an lOl'llilji. higher valency state such as europi-um,which is unstable-at elevated temperatures, from being reduced to alower valency state. Similarly; for convenience, atmospheric pressure isnormallyused, although pressure is not critical. As is well known,increased pressuresin general enhance solubility of the solute,'therebypermitting lower temperatures to be used.

After the heating step the molten solution is cooled at a controlledrate of 0.1 C. per hour'to 25 C. per-hour in the same atmosphere used inthe heating stepluntil it solidifies, forming tungstateor molybdatecrystals having fluorescenteuropiurnrions dispersed therein. Thesolidificatio'n point isreadily determinedvisually. Formost' of themolten solutions, cooling to a temperatureof '650 to 850? C.is adequateto cause solidification.

The tungstate or molybdate; crystals in the flux are'then furnace-cooledor quenched to room temperature. The ditungstateflux is removedfrom thetungstate crystals by Washing the crystals with an alkali such as asolution of r o a with the resulting formation of potassium-terbiumtungstate crystals doped with trivalent europium. The

formed crystals had the following composition:

sodium hydroxide. The .dimolybdate flux isremoved from the'rnolybdatecrystals by washing the crystals with .water. V I v Specific examples ofprocedures utilized in the prepara-- ,tion of compositions of theinvention are given below.

In all cases the properties of the resulting compositionswere measuredas previously described andthe measurements plotted in accordancewiththe description in conjunction with FIG. 2. These examples areYto beconstrued as illustrative only and not as limiting in anyway thescopeand spirit of the invention.

Example 1 75.5 grams KgCO 276.2 grams W0 15.4 grams Tb O and 3.7 gramsEu O were dry mixed together. "The mixture was-then heated in a platinumcrucible in 7 air for 16' hours at a temperature of 1120 C. The moltensolution formed was then cooled in air at, a controlled rate of 2.5 C.perhour to a temperature of'700" 'C.

75.5 grams K CO 2762 'grams W0 18.48 grams TbzOg, and 0.74 gram Eu Owere drymixed. The mixture then underwent the same processing asdetailed above,

Karba 'szw r V V Exampl e 3 25.88 grams K CO 104.25 grams W0 and 13.2grams B11 0 were dry mixed. The mixture then underwent the sameprocessing as detailed above, with the resulting formation of potassiumtungstate crystals doped with trivalent europium-L3The;forrnedcrystalsjhad the following composition; 7 a a 7 o.5 o.sW 4

Example- 4 Eu O =and 12.4 grams TbO- were dry mixed. The mixturethenunderwent the same processing asde'tailedabove with the exceptionthat the formed crystals were washed with Water leavingpotassium-terbium molybdate crystals doped with trivalent europium. T heformed crystals had the following composition: a

v v ExampIe S 53.53 grams K co,, 122.25 grams MoO 10.56 grams -Eu O and2.75 gramsTb O were dry mixed. The mixturethen underwent thesameprocessing as detailed above with the resulting formation ofpotassiumierbiummolybdate crystals doped'with trivalent europium. Theformed crystals had the, following composition .lKolsT oi on i '1 Whatis claimed isi 1 l. A composition of matter havingthe empirical formulaK Tb Eu A where A is selectedufrom the group consisting of W0 and {M00and x has a Value of from 0.003 to-O.50. i V

2. A-composition of matter inaccordance with claim 1 wherein x has avalue of from.0.008 to 0.4.

3. A composition of matter in accordance with claim 1 wherein x has avalue of frornn0;0l to 0.2.

'The resulting solids were thenfurnace cooled to room temperature andwashed with hot sodium hydroxide,

References Cited by the-Examiner UNITED STATES PATENTS 2,929,922 3/60Schawlow ct'al. 886l OTHER REFERENCES Kroger: ,Some Aspects of the"Luminescence;- of

Solids, published..by;the El'sevier PubL'CO. Inc.', New

York, l948, pages 29'l'and 297. g

' MAURICE A. .BRINDISI, Primary Examiner; i"

.JoSEPIr a. LIBERMAN, Examiner. p

1. A COMPOSITION OF MATTER HAVING THE EMPIRICAL FORMULA K0.5TB0.5-XEUXAWHERE A IS SELECTED FROM THE GROUP CONSISTING OF WO4 AND M0L4 AND X HASA VALUE OF FROM 0.003 TO 0.50.